The origin of Rivers Erosion:

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Della Marsh
5 years ago
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1 The origin of Rivers Erosion: is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transport it away to another location. The particulate breakdown of rock or soil into clastic sediment is referred to as physical or mechanical erosion; this contrasts with chemical erosion, where soil or rock material is removed from an area by its dissolving into a solvent (typically water), followed by the flow away of that solution. Eroded sediment or solutes may be transported just a few millimetres, or for thousands of kilometres.

2 Erosion Land degradation by the exogen forces Soil erosion Erosion by fluvial systems Wind erosion Freeze/Thaw erosion Mass movements and landslides Areal erosion Linear erosion

3 Areal Erosion Rain drop erosion Crumb disintegration as a result of the mechanical impact of raindrops Drop erosion is reduced by the vegetation on the surface. The higher the crop coverage is, the more water the foliage can retain, thereby reducing the impact energy and protracting the contact between the drops and the soil surface. Sheet erosion once the upper soil layer reached its maximum water capacity, it is no longer capable to let water through at the same rate as it reaches the surface and a veillike coverage will be developed in the area The accumulated water begins to move towards the slope direction, it forms streamlets and flows towards the next dip

4 Rain drop (splash) erosion Aspects affecting the extent of rain drop erosion: the size of water drops from above amount of water in a unit of time, i.e., the intensity of precipitation duration of rainfall moisture conditions of the soil, water uptake, permeability and retention ability of the soil soil structure and compactness extent of crop coverage

5 Sheet Erosion Extents are less then centimeters Mainly on bare surfaces with gentle roughness Laminar flows Mechanical effect is weak, chemical processes more dominant

6 Linear Erosion Runoff starts when rainfall intensity exceeds soil absorption capacity Runoff starts when all the pores in the soil are filled with water Turbulent flows appear

Detachment in a rill occurs if the sediment in the flow is below the amount the load can

7 Rill erosion Rill erosion is the removal of soil by concentrated water running through little streamlets, or headcuts. Detachment in a rill occurs if the sediment in the flow is below the amount the load can transport and if the flow exceeds the soil's resistance to detachment. As detachment continues or flow increases, rills will become wider and deeper.

Gully erosion occurs when water is channelled across unprotected land and washes away the soil along the drainage lines.

8 Gully erosion Gully erosion is the removal of soil along drainage lines by surface water runoff. Once started, gullies will continue to move by headward erosion or by slumping of the side walls unless steps are taken to stabilise the disturbance. What causes gully erosion? Gully erosion occurs when water is channelled across unprotected land and washes away the soil along the drainage lines. Under natural conditions, run-off is moderated by vegetation which generally holds the soil together, protecting it from excessive run-off and direct rainfall. Excessive clearing, inappropriate land use and compaction of the soil caused by grazing often means the soil is left exposed and unable to absorb excess water. Surface run-off then increases and concentrates in drainage lines, allowing gully erosion to develop in susceptible areas.

9 Stabilising gullies The object is to divert and modify the flow of water moving into and through the gully so that scouring is reduced, sediment accumulates and revegetation can proceed. Stabilising the gully head is important to prevent damaging water flow and headward erosion. A variety of options can be used to get the water safely from the natural level to the gully floor. Improvements like grass chutes, pipe structures, rock chutes or drop structures can be installed to do this effectively. Structures might also be required along gully floors since some grades can be quite steep and allow water to rush down under peak flows, ripping away soil and vegetation. These may take the form of rock barrages, wire netting or logs across gullies. Sediments held in the water will then be deposited along the flatter grades as a result of slower water flow, allowing vegetation to re-establish. If erosion control and revegetation work is undertaken, then damaged areas should be fenced off from stock, until restoration is complete. Dams can also be constructed to slow the flow of water into the gully head, but special care needs to be taken to get the overflow water back into the gully floor safely.

10 Preventing the problem As with other forms of erosion, prevention is better than cure. In most cases gullies can be prevented by good land management practices aimed at maintaining even infiltration rates and a good plant cover. Strategies for preventing gully erosion include: maintaining remnant vegetation along drainage lines and eliminating grazing from these areas increasing water usage by planting deep-rooted perennial pastures, trees, or an appropriate mixture of both thus maintaining healthy, vigorous levels of vegetation identifying drainage lines as a separate land class in which vegetation needs to be protected immediate stabilisation of sheet or rill erosion vermin control ensuring run-off from tracks is evenly distributed across paddocks ton dissipate its energy maintaining high levels of organic matter in the soil avoiding excessive cultivation.

11 Badlands Badlands are a type of dry terrain where softer sedimentary rocks and clay-rich soils have been extensively eroded by wind and water. They are characterized by steep slopes, minimal vegetation, lack of a substantial regolith, and high drainage density. They can resemble malpaís, a terrain of volcanic rock. Canyons, ravines, gullies, buttes, mesas, hoodoos and other such geologic forms are common in badlands. They are often difficult to navigate by foot. Badlands often have a spectacular color display that alternates from dark black/blue coal stria to be bright clays to red scoria.

14 Summary of Erosion types

15 Summary of Erosion types Splash erosion The forece of falling irrigation or rainwater displaces soil particles. Sheet erosion Impermeable surfaces, compacted soil, or bare soil lets water run across it, washing away disturbed surface particles. Rill erosion Sheet erosion wears down soil to establish a define path, forming rivulets in the soil referred to as rills. Rill erosion is much more visible to humans than splash and sheet erosion. Gully erosion Over time, rills widen and deepen into a gully, accelerating the effects of erosion by creating more and more surface area susceptible to disturbance, Bank erosion Fast water flows (often caused by influx of stormwater from impermeable surfaces wear away stream sides at an accelerated pace, often causing bank failure.

16 Stream flow and sediment transport Streams move downslope under the influence of gravity, the passage of water is called stream flow. Several factors control the amount of sediment that can be carried by a stream: 1) volume of stream flow, 2) the stream gradient, 3) shape of the stream channel, 4) kinds and volume of sediments available for erosion in a drainage basin. During floods, the volume and rate of stream flow increases, and erosion along the stream bed mobilizes sediments that accumulate during times of decreasing stream flow. Erosion carves the sides of stream channels, contributing sediments to streams and allowing the channel to migrate over time. Turbulence in the often violent or unsteady movement and mixing of air or water, or of some other fluid.; a most important factor influencing sediment transport in a stream. stream capacity the theoretical maximum mass of suspended sediment transported by a stream difficult to determine because a sediment laden stream is transitional to a debris flow increases with the 2-3rd power of discharge (i.e. faster than the increase of channel width or depth with discharge) as mass wasting and slope erosion in headwaters deliver sediment to tributary streams

17 Stream transport Sediment load in the channel depends on: Physical geography factors Type of input: Direct (rock falls) Indirect (areal erosion, gully erosion, landslides) Transport capicity depends on: Sediment grain size Stream turbulence Stream velocity

19 types of sediment load 1. dissolved sediment in solution (ionic) transport requires no mechanical energy reflects solubility of rocks in the watershed, rates of weathering and proportion of groundwater input versus softer surface water 2. traction (bedload) coarse fraction that rolls and slides along bed or moves in long low paths by saltation common where coarse materials are delivered to the channel with high velocities (flood flows or steep channels) 3. suspended accounts for most stream sediment and most of the work performed by streams, because suspendable (fine) sediments are always available and all streams are capable of suspending fine sediment measured and expressed as load (mass) or concentration (mass/unit volume) concentration decreases downstream as the number of small tributaries (sources of much sediment) decrease and the proportion of baseflow (groundwater) increases

20 Channel geometry significant difference between bedrock (structurally controlled) versus alluvial(adjustable) channels plan view meandering sinuous single thread, the most stable and efficient channel geometry (least variable energy distribution) to conduct water and sediment over any surface (e.g. supraglacial streams formed and maintained by erosion of banks and deposition on point bars braided multiple thread, superimposed meandering channels as discharge and sediment load vary seasonally and diurnally, e.g. semiarid and proglacial streams bars reforms during flood stage, deposition during falling stage that splits subsequent flow different hydraulic geometry at different stages anatomosing permanent multiple channels and mid-channel bars channel width increases and depth decreases below a threshold for sediment transport and flow splits into deeper more narrow channels straight either artificial or structurally controlled

21 longitudinal profile concave upward, i.e. slope is an inverse exponential function of discharge (Q 0.5 to -1.0 ) because flow is more efficient in larger channels, i.e. requires less slope to maintain velocity and sediment transport thus in an influent stream, where Q decreases with distance downstream (e.g. an irrigation canal) slope must increase downstream to maintain flow straight or convex segment of the long profile (e.g. bedrock outcrops, waterfalls, other knickpoints represent local deviations from the gross concave upward profile along the profile potential energy (mgh) is converted to kinetic energy (1/2mv 2 )according to he rate of decrease in h (i.e. the slope) and increase in velocity, until PE reaches 0 at base level: sea level (ultimate base level) or a lake or trunk stream (local base level)

23 meandering straight Braided, anastomosing

25 Meander geometry Channel planform morphometry

26 2014. AUG SEPT AUG AUG

27 Crevasse Splay A crevasse splay is a sedimentary fluvial deposit which forms when a stream breaks its natural or artificial levees and deposits sediment on a floodplain. A breach that forms a crevasse splay deposits sediments in similar pattern to an alluvial fan deposit. Once the levee has been breached the water flows out of its channel. As the water spreads onto the flood plain sediments will start to fall out of suspension as the water loses energy. In some cases crevasse splays can cause a river to abandon its old river channel, a process known as avulsion. Breaches that form a crevasse splay deposits occur most commonly on the outside banks of meanders where the water has the highest energy. Crevasse splay deposits can range in size. Larger deposits can be 6 m (20 ft) thick at the levee and spread 2 km (1.2 mi) wide, while smaller deposits may only be 1 cm (0.39 in) thick.

29 Crevasse Splay

32 Structure of the floodplain

33 Accumulation Alluvial fans

Erosion Surface Water. moving, transporting, and depositing sediment.

Erosion Surface Water. moving, transporting, and depositing sediment. + Erosion Surface Water moving, transporting, and depositing sediment. + Surface Water 2 Water from rainfall can hit Earth s surface and do a number of things: Slowly soak into the ground: Infiltration